1. Field of the Invention
The present invention is directed to the field of sheet material inspection and more specifically to the area of optical flaw detection in float-formed glass and the like.
2. Description of the Prior Art
Several prior art systems have been devised for the purpose of detecting optical transmission flaws which occur in glass at the time of its manufacture. Flaws, such as bubbles, dirt, viscous lumps and stones are generally detectable in prior art systems.
In commonly assigned U.S. Pat. No. 3,493,769 and U.S. Pat. No. 3,743,431, plate glass is scanned by a light beam which is point focused onto a surface of a glass sheet along a path orthogonal to the surface and is reflected back onto itself by a mirror on the opposite side of the glass. The '769 patent illustrates the use of a dark channel detection system whereby a perturbation or scattering of the incident and return beams will result in a detection. The '431 patent teaches the use of both a light and dark channel detection system whereby return light from unflawed material is detected in the light channel and perturbation or scattered light caused by a flaw is detected in the dark channel. The addition of the light channel is effective to increase the capability of the system in order to detect the occurrence of nonscattering occluding flaws.